1. Field of the Invention
The present invention relates to accurately synchronizing physically separated and electrically isolated clocks used to time-stamp data taken during seismic surveys.
2. Description of the Related Art
It has long been the practice to log wells, that is, to sense various downhole conditions within a well and transmit the acquired data to the surface through a wireline or cable-type equipment. To conduct such logging operations, however, drilling was stopped, and the drill string removed from the well. Since it is costly to stop drilling operations, the advantages of logging while drilling have long been recognized. However, the lack of an acceptable telemetering system has been a major obstacle to successful logging while drilling.
Various telemetering methods have been utilized for logging while drilling. For example, it has been proposed to transmit the acquired data to the surface electrically. Such methods have in the past proved impractical because of the need to provide the drill pipe sections with a special insulated conductor and means to form appropriate connections for the conductor at the drill pipe joints. Other techniques include the transmission of acoustical signals through the drill pipe. Examples of such telemetering systems are shown in U.S. Pat. Nos. 3,015,801 and 3,205,477. In those systems, an acoustic energy signal is sent up the drill pipe and frequency modulated in accordance with a sensed downhole condition.
Other telemetering procedures used during logging-while-drilling operations have used the drilling liquid within the well as the transmission medium. U.S. Pat. No. 2,925,251 discloses a system in which the flow of drilling liquid through the drill string is periodically restricted to cause positive pressure pulses to be transmitted up the column of drilling liquid to indicate a downhole condition. U.S. Pat. No. 4,078,620 discloses a system in which drilling liquid is periodically vented from the drill string interior to the annular space between the drill string and the borehole of the well to send negative pressure pulses to the surface in a coded sequence corresponding to a sensed downhole condition. A similar system is described in the Oil and Gas Journal, Jun. 12, 1978, at page 71. Wireless systems have also been proposed using low frequency electromagnetic radiation transmitted through the drill string, the borehole casing, and the earth""s lithosphere to the surface of the earth.
Although the wireless transmission systems just discussed have the potential for increasing the efficiency of drilling operations to offset high operating costs, they are all subject to the disadvantages of transmitting information at a relatively slow rate compared to conventional wireline systems, and are subject to inaccuracies because of the high level of noise usually present in drilling operations. Some seismic systems utilize a drill bit as a seismic source of energy received by seismic receivers on the surface. These systems, however, suffer from errors induced by a drift in separate surface and downhole timing clocks which are utilized to synchronize and correlate data collection. Thus, there is a need for increased accuracy timing to support, for example, a geophysical survey taken during a measurement-while-drilling (xe2x80x9cMWDxe2x80x9d) operation. Clocks typically require synchronization to within approximately 2 milliseconds to achieve the desired accuracy for measurement of the time intervals between activation of a seismic event, for example, a seismic signal source transmitting energy at the earth""s surface and detecting the transmitted energy at a downhole receiver.
Principles of data recording and transmission for MWD instrumentation are explained in U.S. Pat. No. 4,216,536. MWD instruments typically remain in a borehole for several days between drill bit changes. During those several days, clocks have a tendency to drift. Drift is a less significant problem in regular well logging measurements where well logging measurements can be out of synchronization by several minutes without affecting the integrity of the data. Seismic surveys (referred to as xe2x80x9cCheckshotxe2x80x9d), however, require rigorous synchronization of the surface and downhole clocks. U.S. Pat. No. 4,829,489, issued to Rector, describes a method for determining travel time using seismic inputs from a drill bit at the end of a drill string to the surface. The ""489 patent method uses the drill string as an energy source that is inherent to an MWD operation.
Checkshot or seismic surveys inherently require more accurate timing than general logging due to post collection data processing restrictions. U.S. Pat. No. 5,555,220, issued to Minto, describes a receiver conveyed on a slick line that can be dropped or pumped into a drill pipe for taking checkshot or seismic surveys. The ""220 device, however, is limited in the time that the receiver can be left downhole. The surface close and the downhole clock become unsynchronized, necessitating the downhole receiver being removed from the borehole for the sole purpose of synchronizing clocks. In U.S. Pat. No. 5,720,355 issued to Lamine, describes a method for ensuring accurate data collection via electrical communication between physically separated devices.
Thus there is a need for a method and apparatus that enables synchronization of isolated downhole and surface clocks during monitoring-while-drilling operations.
The present invention provides a structure embodying an apparatus and provides a method which enables synchronization of isolated clocks. The present invention synchronizes two or more physically and electrically isolated clocks, a surface clock and a downhole clock. The clocks are not directly electrically connected except for a possibly conductive drill string. The present invention provides for synchronization of downhole and surface clocks which enhances the accuracy and integrity of seismic data gathered during MWD operations.
The present invention sends a synchronization pulse down the borehole via the drill string. A surface clock is referenced to determine the transmit time for a synchronization pulse which as it is transmitted from the surface. A downhole receiver detects the synchronization pulse. A downhole clock is referenced to determine the time at which the synchronization pulse is received at the downhole receiver. The downhole receipt time is later transmitted to the surface via pulses communicated through the drilling mud or fluid. The synchronization pulse is then reflected from the end of the drill string, at or near the downhole receiver and drill bit, back up the drill string to the surface. The surface clock is then referenced to determine the time at which the synchronization pulse is received at the surface.
The synchronization pulse transmit time is then subtracted from the reflected pulse receipt time to determine the round trip time of the synchronization pulse. The synchronization pulse round trip travel time is divided in half to determine the one-way travel time from the surface to the downhole receiver. This calculated one-way synchronization pulse travel time is added to the synchronization pulse transmit time to calculate when the synchronization pulse should have been received at the downhole receiver. The calculated downhole receipt time is calculated at the surface and references the surface clock. The recorded receipt time is recorded by the downhole receiver and references the downhole clock. The difference between the calculated downhole receipt time and the recorded downhole receipt time represents the difference between the surface clock and the downhole clock. The downhole and surface clocks are then synchronized based on the difference between the calculated downhole receipt time and the recorded downhole receipt time.
Synchronization of the downhole and surface clocks enables a passive seismic receiver to remain downhole for extended periods of time without the need to interrupt operations to retract the seismic receiver, as necessary in the past prior to introduction of the present invention. A seismic receiver, utilizing the present invention can now remain downhole until, for example, it is time to change drill bits. Thus, the duration of maintenance of the seismic receiver in a downhole position may comprise periods of days at a time. The present invention enables precise timing in combination with an existing auto correlation procedure, which enables the use of alternative methods of providing a synchronization pulse to the drill string thereby minimizing the chance of damaging the drill string due to physical impact of generating a synchronization pulse.
The present invention provides a structure and a method that enables simplified data manipulation through enhanced timing accuracy. The structure and method of the present invention also enables utilization of components that are less expensive to manufacture than conventional seismic-while-drilling systems. The present invention enables the use of PDC drill bits while conducting a seismic survey during an monitoring-while-drilling operation and can also be used in a deep water environment where conventional seismic receivers are difficult to deploy.
In another aspect of the present invention a method is provided for recording a seismic survey over extended time intervals using a seismic source near a borehole comprising the detection of energy within the borehole, storing the detected energy and correlating the time of storage with an event represented by time stamps stored at a location remote from the place of storage of the energy stored.
In another aspect of the present invention a method is provided for making accurate seismic measurements, comprising synchronizing events time-stamped by first and second clock circuitry using acoustic energy propagated along a physical component within a borehole and correlating seismic and other data using a procedure depending, at least partially, on synchronization between the first and second clock circuitry.
Examples of the more important features of the invention thus have been summarized rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto.